1. Field of the Invention
The present invention relates to a system and method for managing a power source in a vehicle.
2. Background Art
With the ever increasing need to produce vehicles that are more fuel efficient, hybrid electric vehicles (HEV's) have provided an improvement in fuel economy over many conventional vehicles that utilize only an internal combustion engine to drive the vehicle. One of the primary advantages of an HEV is that it allows the vehicle to be powered by an electric motor under certain operating conditions. For example, if the speed of the vehicle is relatively moderate, and the battery or other electrical power source is sufficiently charged, the engine may be shut off, and the vehicle powered exclusively by one or more electric motors. As operating conditions change, the engine may be started to provide additional power, and if needed, charge the battery.
An electrical power source, such as a battery, may be charged and discharged many times over the span of its useful life. Although in theory it may be possible to charge the battery to 100% of its capacity, and discharge the battery until there is zero charge remaining, in practice, it may be beneficial to limit the amount by which the battery can be charged and discharged. For any given power source, such as a battery, the charge and discharge limits may change according to operating conditions. For example, the ability of a battery to accept a charge, or to be discharged, may be dependent on a number of factors, such as the temperature of the battery and the battery state-of-charge (SOC). Moreover, if the battery is made up of a number of individual cells, the voltage of the cells may be another factor that affects the ability of the battery to accept a charge or to be discharged. Therefore, the charge and discharge limits placed on a battery may change as a function of, for example, the battery temperature. Specifically, at relatively low and relatively high battery temperatures, it may be desirable to reduce the charge and discharge limits to reflect the battery's reduced capacity at these temperatures.
In the case of an HEV, which may be powered by an electric motor, an engine, or some combination of the two, a reduced battery discharge limit means that operation of the motor will be limited, and a greater reliance on the engine will be necessary. Of course, increased use of the engine increases the use of fuel, thereby reducing the benefit otherwise obtained by operating an HEV. Thus, at relatively low or relatively high battery temperatures, the battery discharge limit may be met or exceeded with very little output from the motor. When the discharge limit is approached, the engine is started, and use of the motor is prohibited or reduced. This helps to keep the battery temperature under control, and further, helps to keep the battery from being damaged by over discharge.
Setting a discharge limit for a battery in an HEV, and appropriately controlling the electrical loads, including the motor, when the discharge limit is approached, can help increase battery life, by ensuring that the battery does not overheat or over discharge. One problem with this approach is that conventional discharge limits are set without considering very short term, transient outputs of the motor. For example, using a motor in an HEV to start the engine may require approximately 0.5 seconds of motor use. A conventional discharge limit for a battery may be set based on continuous use of a motor for two or more seconds. Thus, a conventional battery discharge limit may be set lower than necessary for purposes of starting an engine. In such a case, the engine may be started sooner than is required, thereby unnecessarily using fuel. In addition, use of a conventional battery discharge limit may cause an engine to remain on in an HEV, when it might otherwise be able to be shut off, thereby increasing fuel economy.
Therefore, a need exists for a system and method for managing a power source in a vehicle that sets a discharge limit for the power source at a level that facilitates reduced use of an internal combustion engine, thereby increasing overall fuel economy, while still protecting the battery from overheating and over discharging.